The central hypothesis of this proposal is that mutations and gene rearrangements that affect gene dosage of the hematopoietic transcription factor RUNXI cooperate with activating mutations in FLT3, KIT and RAS to cause acute myeloid leukemia. During the previous funding period for this proposal, we used a positional cloning strategy to demonstrate that mutations in the hematopoietic transcription factor R UNXI are responsible for the familial platelet disorder with propensity to develop acute myeloid leukemia (FPDIAML syndrome, MIM 601399). Subsequently, we and others have demonstrated similar loss of function mutations in RUNXI in sporadic cases of acute myeloid leukemia. Analysis of several FPD/AML pedigrees supports haploinsufficiency of R UNXI as the cause of the FPD/AML syndrome. However, in sporadic cases of leukemia, both alleles of R UNXI may be mutated, indicating that complete loss of function of RUNXI may contribute to progression to AML. In addition, several pedigrees and sporadic cases of AML harbor RUNXI mutations that may be partially functional or have transdominant effects. Against this backdrop, it has recently been appreciated that increased dosage of RUNX] may also contribute to pathogenesis of hematopoietic neoplasia. In this proposal, we will explore the role of RUNX] dosage effects in the pathogenesis of human leukemia. We anticipated that this analysis would be difficult, in part because of abundant evidence that, although mutations and gene rearrangements of RUNX1 are frequent in leukemia, none of these are sufficient to cause AML. We reasoned that mutations affecting RUNX I dosage may impair hematopoietic differentation, but that additional mutations would be required to confer proliferative and/or survival advantage to these cells. We have demonstrated the presence of activating mutations in the FLT3 and c-KIT receptor tyrosine kinases, and in K-RAS, in human leukemias associated with RUNX1 point mutations. Based on these observations we will pursue the following Specific Aims: 1.Characterize the leukemogenicity of loss or gain of function of Runxl in the mouse; 2. Characterize leukemic potential of activating mutations in hematopoietic receptor tyrosine kinases and K-RAS using murine models; and 3. Characterize cooperativity between activating mutations in FLT3, KIT and K-RAS with mutations affecting Runxl gene dosage in murine models of leukemogenesis.